The integrated circuit industry continues to progress in electronic circuit densification and miniaturization. This progress has resulted in increasingly compact and efficient semiconductor devices, which in turn enable the systems into which these devices are incorporated to be made smaller and less power consumptive.
Among the semiconductor device features that are being reduced in size are the vias, which electrically connect a device's various metal layers. A device's metal layers often include conductive planes that conduct power or provide grounding, and also include patterned conductive layers that serve as input/output and control lines for components on the device (e.g., transistors, resistors, capacitors, etc.). Conductive vias with a circular cross-sectional area are most often used to electrically connect these metal layers. These vias are typically drawn square on the design database.
Two important characteristics of a via are its resistance and the I2R losses associated with the structure. Ideally, the via's resistance should be as close to zero as possible, so that the via has high conductivity. In addition, the I2R loss should be as low as possible, which lowers the device temperature and, in turn, lowers resistance even further. A via's resistance and I2R losses depend on the via's physical attributes (e.g., shape, height, width), the materials used to form the via, and the methods used to deposit the conductive materials into the via openings. Basically, vias are formed by etching openings in dielectric layers of a device, and filling those openings with conductive material.
Via openings are currently designed with diameters within the half micron range, and the aspect ratio of the via openings (i.e., the ratio of the opening's height to its diameter) is becoming very high. For example, aspect ratios for via openings currently can be greater than 3. Because of this, it is difficult to deposit, with good coverage, the necessary conducting material in these tiny, narrow, high-sided via openings. As will be described in more detail below, this makes it difficult to form increasingly miniaturized vias that retain a high conductivity.
With regard to the materials used for the vias, aluminum is commonly used because of its good conductivity and ease of deposition. However, pure aluminum is subject to electromigration failures, which occur when the flow of electrons through the material dislodges aluminum atoms and causes an open circuit condition. This results in poor electrical conduction or total failure.
Another problem that occurs during via formation is that, as a result of exposure to the ambient, a native oxide layer can grow in the bottom of a via opening. These layers are highly insulative and can cause a high resistance, which at times can result in via failure. Typically, a diffusion barrier and/or electrical shunt of a material such as titanium nitride, also referred to as Ti N, is used to remove the native oxide layer. Therefore, methods used for filling via openings should be compatible with the currently known methods for depositing diffusion barriers.
With regard to the processes used to fill via openings with conductive material, one of two processes are commonly used: chemical vapor deposition (CVD); and physical vapor deposition (PVD), also known as sputtering. Limitations of the materials that can be deposited with CVD make it impractical for many applications, so PVD is more commonly used.
One problem associated with PVD is that “cusping” may occur during sputtering or reflowing aluminum into high aspect ratio via openings. Cusping occurs as aluminum aggregates at the top of the via opening and overhangs into the center of the via opening. Cusping can block the via opening before a proper thickness of aluminum can be formed at the bottom of the via opening. This results in a partially filled via opening with poor electrical connection characteristics, and can cause total via failure.
One method for forming a via, which addresses some of the problems described above, is described by Honeycutt et al. in U.S. Pat. No. 6,331,482 B1, issued on Dec. 18, 2001. Using the Honeycutt et al. method, a via is formed by first depositing a standard diffusion barrier in a via opening. A layer of germanium is then formed over the diffusion barrier, and aluminum or aluminum alloy is reflow sputtered into the via opening. Although the technique addresses some of the process and material concerns with vias described above, it does not fully address the inherent resistance and I2R loss issues relating to the via's physical configuration (e.g., the via's shape, aspect ratio, and size).
Accordingly, what is needed is a submicron VLSI via with very low resistance and I2R loss. Particularly needed is a via which has higher reliability than prior art vias. Also needed is a method of forming these vias, which reduces the occurrence of cusping, and improves the electrical characteristics and reliability of the vias.